Due to recent advances in the technology of laser generation and detection, laser systems for use in battlefield conditions have become more and more prevalent. These laser systems are employed for target illumination and tracking or for ranging. In a particular battlefield setting, there may be numerous laser illuminators operating simultaneously. These laser illuminators may be both from friendly forces and from enemy forces. In particular, combat troops operating in this environment will be subject to uncontrolled illumination by laser radiation. Because of the great radiated power from these laser radiation sources, these personnel require some eye protection from this laser illumination. Heretofore, two differing types of laser eye protection have been proposed. The first type includes heavily tinted spectacles. The color of these tinted spectacles covers the bandwidth of the expected laser illumination. The laser light is absorbed by the tint in the spectacles, thereby reducing the light intensity reaching the user's eyes within the wavelength band of the tint.
The use of tinted spectacles has several problems. Firstly, it is difficult to obtain dyes having the necessary absorption band to cover the expected wavelengths of laser illumination. In addition, there is a disadvantageous tradeoff between the amount of protection provided and the reduction of visibility at other wavelengths. Because the absorption band of such tinted spectacles does not correspond to the rather narrow bandwidths of the laser illumination, such tinted spectacles necessarily absorb light at a far greater range of wavelengths than necessary to provide protection. In addition, in order to obtain the desired attenuation of the light at the particular laser illumination frequencies, it is necessary to heavily tint the spectacles. As a consequence, spectacles of this type which are tinted heavily enough in order to provide adequate protection also greatly reduce the visibility of the user at other wavelengths.
A second solution to this problem is the use of holographic optical elements. Holographic optical elements include three-dimensional interference fringe patterns which diffract light at specified wavelengths. Holographic optical elements are ordinarily constructed employing laser illumination forming interference fringes within the volume of a photosensitive medium. Upon development of the photosensitive medium, the pattern of the interference fringes is formed within this medium in the form of varying indexes of refraction. When light of certain wavelengths enters such a holographic optical element, it is diffracted by the interference pattern therein. In the case of laser protection eyewear, it is common to form a reflection holographic optical element which reflects incoming radiation at the particular wavelength in a manner making it appear to be a mirror.
Laser protective eyewear formed in this manner have heretofore been constructed employing spherical geometries concentric about an exposure point source used in construction of the holographic optical element. If the exposure point source is located at the position of the center of rotation of the eye, the geometry requires that the eyewear either have a shape concentric about the center of rotation of the eye or the interference fringes will intersect the surface of the eyewear. Because the construction of highly curved eyewear about the center of the eye is difficult, these holographic optical elements have generally been constructed with relatively flat surfaces. This causes the interference fringes, which are concentric with the center of rotation of the eye, to intersect the front and back surfaces of the spectacles at numerous places. This geometry can cause or aggravate a type of noise phenomenon called flare. This source of flare can be eliminated while still employing a relatively flat holographic optical element by locating the exposure point source at the center of curvature of the element, which would be further from the element than the center of rotation of the eye. This technique would provide interference fringes relatively parallel to the surface of the eyewear, thereby reducing the source of flare.
The use of such spherical geometries requires a disadvantageous compromise to provide the necessary angular protection for the user. Because the eye of the user can rotate, the laser protective eyewear must block radiation at a particular wavelength over a range of angles. At any particualr rotated position of the eye harmful radiation can be received from a relatively narrow range of angles through only a small region of the eyewear. This narrow range of angles differs for different rotated positions of the eye, thus the total angular coverage needed is rather broad. Holographic optical elements constructed using spherical geometries are uniform relative to rotation of the eye. Thus the entire holographic optical element must block the particular wavelength throughout the broad range of angles. Known techniques for covering such broad angular ranges also broaden the band of wavelengths blocked. The broadened band of wavelengths reduces the transmissibility of the laser protective eyewear unnecessarily. In addition, the use of concentric geometry in the formation of such holographic optical elements, means that incoming radiation directed normal to the surface of the eyewear is retroreflected, that is reflected directly back toward the source. This causes the eye protection wearer to be a cooperative target to any enemy laser illuminations. Thus, a group of combat troops employing these laser protection spectacles would be readily visible in laser illumination from enemy troops.